1. Field of Invention
The present invention relates to an etching machine. More particularly, the present invention relates to an etching machine that includes installation for supplying an additional biased voltage to the lower electrode of the machine.
2. Description of Related Art
FIG. 1 is a diagram showing the internal structure of a conventional etching machine. As shown in FIG. 1, the etching machine 2 comprises a process chamber 4, an upper radio frequency (RF) matching element 6, an upper RF matching element driver 8, an RF sensor box 10 and a system controller 12.
The process chamber 4 includes an upper electrode 14 and a lower electrode 16. In general, a wafer 18 is placed on top of the lower electrode 16, and the lower electrode is directly connected to a ground. In operation, voltage supplied to the upper electrode is adjusted so that the combined resistance of the upper and lower electrodes is maintained at around 50 .OMEGA..
The upper RF matching element 6 comprises a matching transformer 20, a manual adjustment knob 21, capacitor units 22 and 23, and variable capacitors 25 and 26. The manual adjustment knob 21 is used for controlling the magnitude of change in the matching transformer 20 whose output is sent to the upper electrode 14. The capacitor units 22 and 23 form capacitor assemblies by connecting with their respective variable capacitors 25 and 26 in parallel.
The upper RF matching element driver 8 comprises two motors 27 and 28, which are connected to the variable capacitors 25 and 26 of the upper RF matching element 6, respectively.
The RF sensor box 10 comprises a coil 30, an RF generator 32 and an RF sensor computer display device 34. The RF sensor computer display device 34 receives voltage signals from the RF generator 32 and current signals from the sensing coil 30, and then displays the data. In addition, the designated frequency produced by the RF generator 32 is around 13.56 MHz and the designated resistance between the upper and lower electrodes is about 50 .OMEGA..
The system controller 12 comprises a computer display device 36, a manual adjustment device 38 and a computer system installation 40. In operation, the etching machine 2 is normally controlled by the system computer. Under exceptional circumstances, the machine can be operated using the manual adjustment device 38.
Theoretically, when the lower electrode 16 is connected to a ground and the resistance of the upper electrode 14 is set by the RF generator 32 to 50 .OMEGA., the system will automatically adjust the resistance in the upper electrode 14 to about 50 .OMEGA.. However, in reality, a corresponding bias voltage will be generated in the lower electrode 16 so that the resistance between the upper and lower electrodes will be different from the value of 50 .OMEGA.. Consequently, it is very difficult to control etching operation at the peripheral region between the upper and lower electrode.
Therefore, when a wafer is placed on top of the lower electrode 16 and the etching operation begins, polymers can accumulate at the peripheral region of the wafer. This will lead to a high backside helium flow. When helium loss is serious, etching rate can become highly non-uniform. Hence, portions of the wafer surface may be over-etched, leading to low yield and product wastage.
In light of the foregoing, there is a need to improve the design of an etching machine.